Optical multi-beam scanning device and image forming apparatus

ABSTRACT

It is an object of the present invention to provide an optical multi-beam scanning device and an image forming apparatus which adopt a horizontal synchronization sensor and can suppress displacement in a horizontal scanning direction even when latent images are written on a surface to be scanned by a plurality of light beams tilting with respect to the surface to be scanned. 
     In the optical multi-beam scanning device of the present invention, when the light beams are assumed to reach the surface to be scanned with the light beams not being folded, the horizontal synchronization sensor is tilted so as to output a horizontal synchronized signal when the light beams come to the same position on the surface to be scanned in the horizontal scanning direction. In another method, the horizontal synchronization sensor is not tilted, and the similar function is executed by setting a boundary position between a shielding portion and a non-shielding portion of a light shielding member provided on an upper stream side. The image forming apparatus of the present invention adopts the optical multi-beam scanning device of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.11/594,145, filed Nov. 8, 2006, which is a divisional of U.S.application Ser. No. 10/810,707, filed Mar. 29, 2004, the entirecontents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an image forming apparatus such as a compositemachine which bears the copying function and the printing function of acopying machine and a printer, and an optical multi-beam scanning devicewhich is mounted on the image forming device. Specifically, theinvention relates to the optical multi-beam scanning device for writinglatent images using a plurality of light beams on one photosensitivedrum, and the image forming apparatus.

2. Description of the Related Art

In the case where a surface to be scanned on a photosensitive drum isscanned by an optical scanning device, when a light beam verticallyenters the photosensitive drum, a part of the light beam which entersthe photosensitive drum reflects so as to return to a light deflectingdevice. The reflected light beam is reflected as a secondary reflectedlight so as to return to the surface to be scanned again become a fixedstray light. For this reason, in general, a light beam is allowed toenter the photosensitive drum at an angle which tilts with respect to avertical scanning direction. In this case, one light beam does not arisea problem, but when latent images are written by a plurality of lightbeams, a distance from a deflector to an image surface varies with thelight beams. For this reason, a value “f” of an fθ characteristic varieswith the light beams, and when an image is written with one imagefrequency, a position in a horizontal scanning direction shifts.

In a circle A of FIG. 18, conventional emitting positions of two lightbeams LBa and LBb in the vertical scanning direction are enlarged, andin a circle B of FIG. 18, conventional emitting positions of the twolight beams LBa and LBb at one scanning end of the horizontal scanningdirection are enlarged. Since an optical path from the light beam LBa toa surface to be scanned (the surface of photosensitive drum) SUR isshorter than that from the light beam LBb to the surface to be scanned,a displacement ΔH takes place between the light beams LBa and LBb evenin the horizontal scanning direction at the same deflection angleaccording to a difference ΔD of the optical path lengths as shown in thecircle B of FIG. 18.

Such a method of solving the displacement in the horizontal scanningdirection is disclosed in U.S. Patent Publication No. 2003/0043441 A1.

In the method disclosed in this publication, different wavelengths areapplied to two light beams, and a difference in magnification due to adifference of optical paths is canceled by magnification chromaticaberration of a post-deflection optical imaging system so thatdisplacement is removed. A horizontal synchronized signal is inputdirectly into a sensor without through the post-deflection opticalimaging system, and when the beams come to the same position on thesurface to be scanned, they are detected.

In the method disclosed in the publication, however, write timing of animage is determined based on an output of a horizontal synchronizationsensor for detecting light beams which do not go through thepost-deflection optical imaging system. For this reason, the followingproblems (1) to (3) arise.

(1) In order to allow the light beams which do not go through thepost-deflection optical imaging system to enter the sensor for detectinglight beams to produce the horizontal synchronized signal, it isnecessary to secure a sufficient distance in the horizontal scanningdirection from a light beam which goes through the post-deflectionoptical imaging system so as to be imaged on an image effective area toa light beam for obtaining horizontal synchronization (since an edgeportion of the optical system cannot be used, the light beam forhorizontal synchronization should be made to pass through the outside ofthe edge portion). As a result, it is necessary to enlarge a size of apolygon mirror or decrease a number of surfaces of the polygon mirror.When the size of the polygon mirror is increased, windage increases, andheat generation and noise become large. On the other hand, when a numberof the surfaces of the polygon mirror is decreased, it is necessary toheighten a revolution speed in order to cope with a uniform processspeed. As a result, windage increases, and heat generation and noisebecome large.

(2) An arrangement of optical elements in the post-deflection opticalimaging system or a principal ray emitted from a pre-deflection opticalsystem occasionally deviates from a design value. In this case, a timedifference that a plurality of beams reaches a predetermined position ofthe horizontal synchronization sensor occasionally shifts from a timedifference that the beams reach a predetermined position on the surfaceto be scanned.

(3) A relative position relationship among the beams on the surface tobe scanned is not maintained on the surface of the horizontalsynchronization sensor to which a plurality of beams which do not gothrough the post-deflection optical imaging system are given. That is tosay, the horizontal synchronization sensor cannot be provided with afunction for detecting the beam relative position and cannot make acontrol using an actuator based on the information about the beamrelative position.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an opticalmulti-beam scanning device which can suppress displacement in ahorizontal scanning direction even when a horizontal synchronizationsensor to which a light beam similar to that to be given to a surface tobe scanned is given and latent images are written on the surface to bescanned by a plurality of light beams tilting with respect to thesurface to be scanned, and an image forming apparatus which adopts theoptical multi-beam scanning device.

An optical multi-beam scanning device of the present invention ischaracterized by including: a plurality of light sources; deflectingmeans for deflecting light beams from the light sources; post-deflectionoptical means for making the light beams deflected by the deflectingmeans enter a surface to be scanned in a vertical scanning directionwith respect to a normal direction of the surface to be scanned at apredetermined angle; horizontal synchronization detecting means forsynchronizing the light beams in a horizontal scanning direction; andoptical path folding means for folding the light beams directing towardsthe surface to be scanned to the horizontal synchronization detectingmeans. The optical multi-beam scanning device is characterized in thatwhen the light beams are assumed to reach the surface to be scanned withthe light beams not being folded by the optical path folding means, alight receiving surface of the horizontal synchronization detectingmeans is tilted so as to output a horizontal synchronized signal whenthe light beams come to the same position on the surface to be scannedin the horizontal scanning direction.

Furthermore, an optical multi-beam scanning device of another inventionis characterized by including: a plurality of light sources; deflectingmeans for deflecting light beams from the light sources; post-deflectionoptical means for making the light beams deflected by the deflectingmeans enter a surface to be scanned in a vertical scanning directionwith respect to a normal direction of the surface to be scanned at apredetermined angle; horizontal synchronization detecting means forsynchronizing the light beams in a horizontal scanning direction;optical path folding means for folding the light beams directing towardsthe surface to be scanned to the horizontal synchronization detectingmeans; and a light shielding member having a tilt such that when thelight beams are assumed to reach the surface to be scanned with thelight beams not being folded by the optical path folding means, thelight beams are emitted to a light receiving surface of the horizontalsynchronization detecting means with a uniform rate when the light beamscome to the same position on the surface to be scanned.

Further, an optical multi-beam scanning device of another invention ischaracterized by including: a plurality of light sources; deflectingmeans for deflecting light beams from the light sources; post-deflectionoptical means for making the light beams deflected by the deflectingmeans enter a surface to be scanned in a vertical scanning directionwith respect to a normal direction of the surface to be scanned at apredetermined angle; horizontal synchronization detecting means forsynchronizing the light beams in a horizontal scanning direction; andoptical path folding means for folding the light beams directing towardsthe surface to be scanned to the horizontal synchronization detectingmeans. The optical multi-beam scanning device is characterized in thatan optical element for changing an emitting angle according to afluctuation in the wavelengths of the light beams emitted from the lightsources is arranged on an optical path between the deflecting means andthe horizontal synchronization detecting means.

Further, an optical multi-beam scanning device of another invention ischaracterized by including: a plurality of light sources; deflectingmeans for deflecting light beams from the light sources; post-deflectionoptical means for making the light beams deflected by the deflectingmeans enter a surface to be scanned in a vertical scanning directionwith respect to a normal direction of the surface to be scanned at apredetermined angle; horizontal synchronization detecting means forsynchronizing the light beams in a horizontal scanning direction; andoptical path folding means for folding the light beams directing towardsthe surface to be scanned to the horizontal synchronization detectingmeans. The optical multi-beam scanning device is characterized in thatthe optical path folding means changes an emitting angle according to afluctuation in wavelengths of the light beams emitted from the lightsources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a color image formingapparatus according to a first embodiment of the present invention;

FIG. 2 is a schematic plan view illustrating an optical multi-beamscanning device according to the first embodiment;

FIG. 3 is an explanatory diagram illustrating components of apost-deflection optical system in the optical multi-beam scanning deviceaccording to the first embodiment;

FIG. 4 is an explanatory diagram illustrating components of apre-deflection optical system in the optical multi-beam scanning deviceaccording to the first embodiment;

FIGS. 5A to 5C are diagrams explaining a plurality of beams to bedetected by a horizontal synchronization sensor according to the firstembodiment;

FIGS. 6A to 6C are diagrams explaining a first posture example of thehorizontal synchronization sensor according to the first embodiment;

FIGS. 7A to 7C are diagrams explaining a second posture example of thehorizontal synchronization sensor according to the first embodiment;

FIGS. 8A to 8C are diagrams explaining a third posture example of thehorizontal synchronization sensor according to the first embodiment;

FIGS. 9A and 9B are diagrams explaining a first detecting system of thehorizontal synchronization sensor according to the first embodiment;

FIG. 10 is a diagram explaining a second detecting system of thehorizontal synchronization sensor according to the first embodiment;

FIG. 11 is a schematic plan view illustrating a constitution of a mainportion of the optical multi-beam scanning device according to a secondembodiment;

FIGS. 12A to 12C are diagrams explaining a function of a light shieldingmember according to the second embodiment;

FIG. 13 is a schematic plan view illustrating an optical path between afolding mirror which folds an optical path to the horizontalsynchronization sensor and the horizontal synchronization sensoraccording to a third embodiment;

FIG. 14 is a diagram (1) explaining necessity of an optical pathcorrecting element according to the third embodiment;

FIG. 15 is a diagram (2) explaining necessity of the optical pathcorrecting element according to the third embodiment;

FIG. 16 is a plan view illustrating a specific example of the opticalpath correcting element according to the third embodiment;

FIG. 17 is a schematic plan view illustrating an optical path between areflection type diffraction grating which folds the optical path to thehorizontal synchronization sensor and the horizontal synchronizationsensor in a modified example of the third embodiment; and

FIG. 18 is a diagram explaining a reason that displacement in thehorizontal scanning direction among the beams occurs in a conventionaltechnique.

DETAILED DESCRIPTION OF THE INVENTION

An optical multi-beam scanning device and an image forming apparatusaccording to preferred embodiments of the present invention areexplained below with reference to the drawings.

(A) First Embodiment

FIG. 1 is a diagram illustrating a color image forming apparatus intowhich the optical multi-beam scanning device according to a firstembodiment of the present invention is incorporated. This kind of thecolor image forming apparatus utilizes four sets of various deviceswhich form four kinds of image data separated into color components of Y(yellow), M (magenta), C (cyan), and B (black) and form images for therespective color components corresponding to Y, M, C and B. For thisreason, Y, M, C and B are added to respective reference numerals so thatthe image data for the respective color components and the devices arediscriminated.

As shown in FIG. 1, an image forming apparatus 100 has first to fourthimage forming sections 50Y, 50M, 50C and 50B for forming images forcolor-separated color components.

The image forming sections 50Y, S0M, 50C and 50B are arranged in thisorder below an optical scanning device 1 correspondingly to positionswhere laser beams L (Y, M, C and B) are emitted. The laser beams areused for optically scanning image information about the color componentsusing a first folding mirror 33B and third folding mirrors 37Y, 37M and37C of an optical multi-beam scanning device 1, detailed with referenceto FIGS. 2 and 3.

A conveyance belt 52 for conveying a transfer material on which theimages formed via the image forming sections 50 (Y, M, C and B) aretransferred is arranged below the image forming sections 50 (Y, M, C andB).

The conveyance belt 52 is entrained between a belt driving roller 56 anda tension roller 54 which are rotated to a direction of an arrow by amotor, not shown. The conveyance belt 52 is rotated at a predeterminedspeed to a direction where the belt driving roller 56 rotates.

The image forming sections 50 (Y, M, C and B) have photosensitive drums58Y, 58M, 58C and 58B which are a cylindrical shape rotatable to adirection of an arrow, respectively. Electrostatic latent imagescorresponding to the images exposed by the optical scanning device 1 areformed on the photosensitive drums.

Charging devices 60 (Y, M, C and B), developing devices 62 (Y, M, C andB), transfer devices 64 (Y, M, C and B), cleaners 66 (Y, M, C and B),and discharger 68 (Y, M, C and B) are arranged in this order,respectively around the photosensitive drums 58 (Y, M, C and B) alongthe direction where the photosensitive drums 58 (Y, M, C and B) rotate.The charging devices 60 (Y, M, C and B) provide predetermined electricpotentials to the surfaces of the photosensitive drums 58 (Y, M, C andB). The developing devices 62 (Y, M, C and B) supply toner having colorscorresponding to the electrostatic latent images formed on the surfacesof the photosensitive drums 58 (Y, M, C and B) so as to develop theimages. The transfer devices 64 (Y, M, C and B) are arranged so as to beopposed to the photosensitive drums 58 (Y, M, C and B) on a rear surfaceof the conveyance belt 52 with the conveyance belt 52 interveningbetween the transfer devices and the photosensitive drums. The transferdevices 64 (Y, M, C and B) transfer the toner images on thephotosensitive drums 58 (Y, M, C and B) onto a recording medium conveyedby the conveyance belt 52, namely, recording paper P. The cleaners 66(Y, M, C and B) eliminate residual toner on the photosensitive drums 58(Y, M, C and B) which is not transferred when the transfer devices 64(Y, M, C and B) transfers the toner images onto the paper P. Thedischarger 68 (Y, M, C and B) remove residual potentials which remain onthe photosensitive drums 58 (Y, M, C and B) after the transfer devices64 (Y, M, C and B) transfer the toner images.

A paper cassette 70 for housing the recording paper P onto which theimages formed by the image forming sections 50 (Y, M, C and B) aretransferred is arranged below the conveyance belt 52.

A feeding roller 72 which is primarily formed into a half-moon shape andtakes out the paper P housed in the paper cassette 70 one by onestarting from the top is arranged on one end of the paper cassette 70which is close to a tension roller 54.

A register roller 74 for aligning a forward end of one piece of paper Ptaken out of the cassette 70 with a forward end of the toner imageformed on the photosensitive drum 58B of the image forming section 50B(black) is arranged between the feeding roller 72 and the tension roller54.

An absorption roller 76 is arranged in a position which is in a vicinityof the tension roller 54 between the register roller 74 and the firstimage forming section 50Y and is substantially opposed to an outerperiphery of the conveyance belt 52 corresponding to a position wherethe tension roller 54 contacts with the conveyance belt 52. Theadsorption roller 76 provides a predetermined electrostatic adsorptionforce to one piece of paper P conveyed at predetermined timing by theregister roller 74.

Registration sensors 78 and 80 are arranged on the outer periphery ofthe conveyance belt 52 which is one end of the conveyance belt 52 and ina vicinity of the belt driving roller 56 and substantially contacts withthe belt driving roller 56 with a predetermined distance in an axialdirection of the belt driving roller 56 (since FIG. 1 is a frontsectional view, the first sensor 78 positioned on a front side of thesheet in FIG. 1 cannot be seen). The registration sensors 78 and 80detect positions of images formed on the conveyance belt 52 or positionsof images transferred onto the paper P.

A conveyance belt cleaner 82 is arranged in a position which is on theouter periphery of the conveyance belt 52 contacting with the beltdriving roller 56 and does not contact with the paper P conveyed by theconveyance belt 52. The conveyance belt cleaner 82 removes toner or slipof paper adhering to the conveyance belt 52.

A fixing device 84 is arranged in a direction where the paper P conveyedby the conveyance belt 52 is separated from the belt driving roller 56and is further conveyed. The fixing device 84 fixes the toner imagestransferred onto the paper P to the paper P.

FIGS. 2 and 3 are diagrams illustrating the optical multi-beam scanningdevice which is incorporated into the image forming device shown in FIG.1.

The optical multi-beam scanning device 1 has light sources 3Y, 3M, 3Cand 3B, and a light deflecting device 7 as deflecting means. The lightsources 3Y, 3M, 3C and 3B output light beams to the first to fourthimage forming sections 50Y, S0M, S0C and 50B shown in FIG. 1,respectively. The light deflecting device 7 deflects (scans) the lightbeams (laser beams) emitted from the light sources 3 (Y, M, C and B)towards imaging surfaces arranged on predetermined positions, namely,outer peripheral surfaces of the photosensitive drums 58Y, 58M, 58C and58B of the first to fourth image forming units S0Y, 50M, 50C and 50Bshown in FIG. 1 at a predetermined linear speed. Pre-deflection opticalsystems 5 (Y, M, C and B) are arranged between the light deflectingdevice 7 and the light sources 3 (Y, M, C and B). A post-deflectionoptical system 9 is arranged between the light deflecting device 7 andthe imaging surfaces.

A direction where the light deflecting device 7 deflects (scans) thelaser beams is called as a horizontal scanning direction. A direction,which crosses perpendicularly to the horizontal scanning direction andan axial line as a basis of a deflecting operation which is performed onthe laser beams by the light deflecting device so that the laser beamsscanned (deflected) by the light deflecting device direct to thehorizontal scanning direction, is called as a vertical scanningdirection.

The light sources 3 (Y, M, C and B) for the respective color componentsare constituted so that four pairs of semiconductor laser elements 3Yaand 3Yb, 3Ma and 3Mb, 3Ca and 3Cb, and 3Ba and 3Bb are arranged inpredetermined positions.

In the pre-deflection optical systems 5, the laser beams LYa and LYb,LMa and LMb, LCa and LCb, and LBa and LBb emitted from the pairedsemiconductor laser elements 3Ya and 3Yb, 3Ma and 3Mb, 3Ca and 3Cb, and3Ba and 3Bb for the respective color components are synthesized into oneoptical path of each color component by group synthesizing optical parts15Y, 15M, 15C and 15B for fixing up the same color components into oneoptical path. Further, the optical paths of each color component aresynthesized into one optical path by color synthesizing optical parts19M, 19C and 19B, and the laser beams L{(LYa+LYb)=LY, (LMa+LMb)=LM,(LCa+LCb)=LC, and (LBa+LBb)=LB} synthesized in such a manner are guidedto the light deflecting device 7. Before the laser beams LYa, LMa, LCaand LBa emitted from the lasers 3Ya, 3Ma, 3Ca and 3Ba composing therespective light sources are synthesized with the laser beams LYb, LMb,LCb and LBb by the group synthesizing optical parts 15Y, 15M, 15C and15B, reflecting angles of corresponding galvano mirrors 18Y, 18M, 18Cand 18B are set to predetermined angles. As a result, intervals in thevertical scanning direction are set to predetermined intervals.

As shown in FIG. 4 (arbitrary laser beam L is shown as representative),the pre-deflection optical system 5 has a finite focal lens 13, adiaphragm 14, a group synthesizing optical part 15, and a cylinder lens17. The finite focal lens 13 gives a predetermined focusing property tothe laser beam L emitted from the semiconductor laser element 3. Thediaphragm 14 gives an arbitrary sectional beam shape to the laser beam Lwhich passes through the finite focal lens 13. The cylinder lens 17further gives a predetermined focusing property to the laser beam Lsynthesized by the group synthesizing optical part 15 in the verticalscanning direction. The pre-deflection optical system 5 shapes thesectional beam shape of the laser beam L emitted from the laser 3 into apredetermined shape so as to guide the laser beam L to the reflectingsurface of the light deflecting device 7. In FIG. 4, the galvano mirror18 and the color synthesizing optical part 19 are omitted.

The light deflecting device 7 has a polygon mirror 7 a whose, forexample, eight plane reflecting surfaces (plane reflecting mirror) arearranged into a regular polygon shape and a motor 7 b for rotating thepolygon mirror 7 a at a predetermined speed to the horizontal scanningdirection.

The post-deflection optical system 9 has a pair of imaging lenses 21 (21a and 21 b), a horizontal synchronization sensor, a horizontalsynchronization folding mirror 29, a plurality of mirrors 33Y, 35Y and37Y (Yellow), 33M, 35M and 37M (magenta), 33C, 35C and 37C (Cyan), and33B (black), and the like. The pair of imaging lenses 21 optimizesshapes and positions of the laser beams L (Y, M, C and B) deflected(scanned) by the polygon mirror 7 a on the imaging surfaces. Thehorizontal synchronization sensor detects the laser beams or arepresentative laser beam (for example, LB) in order to conform thehorizontal synchronization of the laser beams L (Y, M, C and B) passingthrough the pair of imaging lenses 21. The horizontal synchronizationfolding mirror 29 folds the laser beams L towards the horizontalsynchronization sensor 23. The mirrors 33Y, 35Y, 37Y, 33M, 35M, 37M,33C, 35C, 37C and 33B guide the laser beams L (Y, M, C and B) for therespective color components emitted from the pair of imaging lenses 21to the corresponding photosensitive drums 58 (Y, M, C and B).

In the first embodiment, as mentioned later, the horizontalsynchronization sensor 23 is different from a conventional one, and itis characterized by installing it in a slanted manner.

FIGS. 5A to 5C are enlarged diagrams illustrating a state that widths ofscanning lines of the two laser beams (here, LBa and LBb) for the samecolor component to be used for detection of the horizontalsynchronization at the same deflecting angle are aligned. FIG. 5A is theenlarged diagram in the vertical scanning direction illustrating theenlarged emitting positions of the laser beams LBa and LBb (optical pathindicated by a broken line) on the photosensitive drums 58 which areshown by developing folding of the optical path by means of the foldingmirror 29 for guiding beams to the horizontal synchronization sensor 23and folding of the optical paths by means of the mirrors 33Y, 35Y, 37Y,33M, 35M, 37M, 33C, 35C, 37C, and 33B on FIG. 3. FIGS. 5B and 5C are theenlarged diagrams in the horizontal scanning direction illustrating theemitting positions of the laser beams LBa and LBb (optical path of chiefray indicated by a chain line) on the photosensitive drums 58 invicinities of a maximum deflecting angle and a minimum deflecting angleshown by developing folding of the optical path by means of the foldingmirror 29 for guiding the beams to the horizontal synchronization sensor23 and folding of the optical paths by means of the mirrors 33Y, 35Y,37Y, 33M, 35M, 37M, 33C, 35C, 37C and 33B on FIG. 2. FIGS. 5B and 5Ccorrespond to a case where the horizontal synchronization sensor 23 istilted (posture) shown in FIG. 6, mentioned later.

In FIG. 5A, an up-down direction is the vertical scanning direction, anormal direction is the horizontal scanning direction, and a left-rightdirection is a direction which crosses perpendicularly to the horizontalscanning direction and the vertical scanning direction (hereinafter, athird direction). In FIGS. 5B and 5C, an up-down direction is thehorizontal scanning direction, a normal direction is the verticalscanning direction, and a right-left direction is the third direction.

In the first embodiment, as shown in FIG. 5A, the horizontalsynchronization sensor 23 is tilted so that even when the two laserbeams LBa and LBb which tilt to the normal direction of thephotosensitive drums 58 enter the photosensitive drums 58, the positionsin the horizontal scanning direction of the laser beams LBa and LBbwhose detected signals are output from the horizontal sensor 23 arealigned.

For example, the horizontal synchronization sensor 23 is placed in aposition equivalent to the imaging surface with the optical path lengthsof the laser beams LBa and LBb being aligned. As a result, the positionsin the horizontal scanning direction of the laser beams LBa and LBbdiffer form each other at timing that a horizontal synchronized signalis output.

The laser beams LBa and LBb are in the same position in the horizontalscanning direction on the surface to be scanned in order to avoid theabove inconvenience. At this time, in order that the horizontalsynchronized signals are output, as shown in FIGS. 6A to 6 c, 7A to 7Cor 8A to 8C, an installation posture of the horizontal synchronizationsensor 23 is first determined. A straight line determined by a linewhich connects cross points of the respective beams may be a positionfrom which the horizontal synchronized signal is output.

In FIGS. 6A to 8C, FIGS. 6A, 7A and 8A are diagrams illustrating adetecting surface of the horizontal synchronization sensor 23, and inthose diagrams, an up-down direction is the horizontal scanningdirection, a left-right direction is the vertical scanning direction,and a normal direction is the third direction. In FIGS. 6A, 7A and 8A, athick line indicates a portion which functions for detection(hereinafter, detection stripe), and scanning trajectories of the laserbeams LBa and LBb in the horizontal scanning direction are also written.In FIGS. 6B, 7B, 8B, 6C, 7C and 8C, the detection stripe of thehorizontal synchronization sensor 23 is viewed from another directionand is written. In FIGS. 6B, 7B and 8B, an up-down direction is thehorizontal scanning direction, a right-left direction is the thirddirection, and a normal direction is the vertical scanning direction. InFIGS. 6C, 7C and 8C, an up-down direction is the vertical scanningdirection, a right-left direction is the third direction, and a normaldirection is the horizontal scanning direction.

The example shown in FIGS. 6A to 6C shows the case where the detectionstripe has the same tilt as that in the vertical scanning direction onthe photosensitive drum as the surface to be scanned. In this case,since the optical path lengths of the laser beams LBa and LBb are thesame as those on the photosensitive drums, the positional relationshipsin the horizontal scanning direction and the vertical scanning directionbetween the laser beams LBa and LBb on the surface to be scanned are thesame. The example shown in FIGS. 7A to 7C is the case where thedetection stripe does not have the tilt in the vertical scanningdirection. The detection stripe has the tilt in the horizontal scanningdirection so that when the laser beams LBa and LBb come to the sameposition in the horizontal scanning direction, the laser beams cross thedetection stripe. The example shown in FIGS. 8A to 8C is the case of anintermediate example of the examples shown in FIGS. 6A to 7C, and thedetection stripe tilts in all the directions.

FIGS. 9A and 9B are explanatory diagrams illustrating a first detectingsystem of the horizontal synchronization sensor 23. When a certain beam(beam spot) moves linearly on the photoelectric sensor (horizontalsynchronization sensor) 23 as shown in FIG. 9A, the output afterphotoelectric conversion changes as shown in FIG. 9B. A predeterminedposition can be defined at a rise edge of the output. In the case ofthis detecting system, therefore, a left edge portion (thick lineportion) shown in FIG. 9A corresponds to the detection stripe in FIGS.6A to 8C.

FIG. 10 is an explanatory diagram illustrating a second detecting systemof the horizontal synchronization sensor 23. As shown in FIG. 10, twophotoelectric sensors 23-1 and 23-2 are provided with a gap smaller thanthe diameter of the beam spot. In this case, when a certain beam (beamspot) moves linearly and the center of the beam spot comes to the gapbetween the photoelectric sensors 23-1 and 23-2, outputs from thephotoelectric sensors 23-1 and 23-2 match with each other. As a result,the predetermined position can be defined. In the case of this detectingsystem, therefore, the gap between the photoelectric sensors 23-1 and23-2 shown in FIG. 10 corresponds to the detection stripe explained withreference to FIGS. 6A to 8C.

In the tilted arrangement example of the horizontal synchronizationsensor 23 shown in FIGS. 6A to 6C, the position relationship between thelaser beams LBa and LBb similar to that on the surface to be scanned canbe reproduced on the horizontal synchronization sensor 23. For thisreason, it is effective that this example is applied to the case where abeam position detecting function is provided to the horizontalsynchronization sensor 23, and its result is fed back to an actuator(for example, the actuator of the galvano mirrors 18Y, 18M 18C and 18B).

The tilted arrangement examples of the horizontal synchronization sensor23 shown in FIGS. 7A to 8C are effective for the case where theinformation about the beam relative position in the vertical scanningdirection is not necessary. The tilted arrangement example in FIGS. 7Ato 7C is effective for the case where a sensor surface is set to bevertical to the optical axis, for example, the case where the housing ismade by aluminum die casting and a fixed standard surface of thehorizontal synchronization sensor is shaved by a post-process. Thetilted arrangement example in FIGS. 8A to 8C is, on the contrary,effective for the case where a predetermined tilt is given in thevertical scanning direction, for example, the case where when apredetermined gradient (draft taper) is given to the standard surface asa molding housing, the tilt angle can be set freely.

In any of the tilted arrangement examples of the horizontalsynchronization sensor 23 shown in FIGS. 6A to 8C, every scanning timemay be standard timing at which a latent image is written by each beam.In areas other than the vertical scanning direction effective area,differences in the time at which respective beams come to the positionof the same horizontal scanning direction on the surface to be scannedmay be measured. When latent images are written actually, the horizontalsynchronization sensor 23 detects every scanning of only one beam, andthe write timing of the other beams may be shifted by the timedifferences.

When the horizontal synchronization sensor 23 is tried to be providedslantingly, position accuracy and angle accuracy of the sensor surfacewith respect to an outer shape of the package for covering the sensorsurface become easily insufficient. This should be taken intoconsideration. In the case where accuracy is not much required, thehorizontal synchronization sensor 23 may be installed so as to have astandard surface which is integral with the housing and push the packageagainst its standard surface.

According to the optical multi-beam scanning device and the imageforming apparatus of the first embodiment, the horizontalsynchronization sensor to which the light beams similar to those givento the surface to be scanned are given is installed slantingly. For thisreason, even when latent images are written on the surface to be scannedby a plurality of light beams which tilt with respect to the surface tobe scanned, displacement in the horizontal scanning direction can besuppressed. As a result, quality of the formed image can be improved.

(B) Second Embodiment

FIG. 11 is a diagram illustrating a constitution of a main section ofthe optical multi-beam scanning device according to a second embodiment.

In the case of the second embodiment, the constitution of the imageforming apparatus into which the optical multi-beam scanning device isincorporated is the same as that in FIG. 1. Meanwhile, the opticalmulti-beam scanning device according to the second embodiment is, asshown in FIG. 11, different from the first embodiment in the followingpoint. A light shielding member 25 is provided between the foldingmirror 29 for guiding light beams to the horizontal synchronizationsensor 23 and the horizontal synchronization sensor 23.

Also in the optical multi-beam scanning device according to the secondembodiment, the other part of the constitution is similar to that in thefirst embodiment. The second embodiment 2 is similar to the firstembodiment in that the post-deflection optical system 9 having thefollowing constitution is premised. A plurality of light beams, whichare deflected by the light deflecting device 7 and transmit through thepost-deflection optical system 9, enter the normal of the surface to bescanned at a predetermined angle in the vertical scanning direction.Moving amounts of the light beams in the horizontal scanning directionwith respect to the same deflection angle of the light deflecting device7 are the same as one another.

As explained in the first embodiment, when the horizontalsynchronization sensor 23 is tried to be installed slantingly, theposition accuracy and the angle accuracy of the sensor surface withrespect to the outer shape of the package for covering the sensorsurface become easily insufficient. The method of providing the lightshielding member 25 is effective for such a situation.

The light shielding member 25 is provided so as to produce the similarstate (equivalent state) to the tilted state of the horizontalsynchronization sensor 23 explained in the first embodiment withoutdepending on the angle of the horizontal synchronization sensor 23.

FIGS. 12A, 12B and 12C are explanatory diagrams illustrating postures ofthe light shielding member 25 which produces the similar state(equivalent state) to the tilted state of the horizontal synchronizationsensor 23 shown in FIGS. 6A to 6C explained in the first embodiment.FIGS. 12A to 12C correspond to FIGS. 6A to 6C. In FIG. 12A, an up-downdirection is the horizontal scanning direction, a right-left directionis the vertical scanning direction, and a normal direction is the thirddirection. In FIG. 12B, an up-down direction is the horizontal scanningdirection, a right-left direction is the third direction, and the normaldirection is the vertical scanning direction. In FIG. 12C, an up-downdirection is the vertical scanning direction, a right-left direction isthe third direction, and a normal direction is the horizontal scanningdirection.

In FIGS. 12A to 12C, an edge of a thick line portion is an edge of aboundary where the horizontal synchronization sensor 23 is changed froma light non receiving state (due to shielding by means of the lightshielding member 25) into a light receiving state. The horizontalsynchronization sensor 23 can detect the predetermined positionaccording to the state change. In this case, the enough large horizontalsynchronization sensor 23 should be prepared so that even when itsinstallation position shifts to a certain degree, the scanned laserbeams reach the light receiving surface of the sensor 23 from the thickline portion of the light shielding member 25.

When the horizontal synchronization sensor 23 has the function fordetecting the beam relative position in the vertical scanning direction,as shown in FIGS. 12A to 12C, the horizontal synchronization sensor 23is arranged so as to have a tile which is the same as that of thesurface to be scanned. As a result, pitches among the beams on thesurface to be scanned and pitches between the beams on the sensorsurface can be matched with each other. When the light shielding member25 is, as shown in FIG. 11, integral with the housing for holding thepost-deflection optical system, an error due to its incorporating can beavoided so that accuracy can be easily obtained.

FIGS. 12A to 12C are the diagrams illustrating the postures of the lightshielding member 25 which produces the similar state (equivalent state)to the tilted state of the horizontal synchronization sensor 23 shown inFIGS. 6A to 6C explained in the first embodiment. The posture of thelight shielding member 25 may be set, however, so as to produce thesimilar states (equivalent state) to the tilted states of the horizontalsynchronization sensor 23 shown in FIGS. 7A to 8C explained in the firstembodiment according to the installation conditions. These states arenot shown.

The technical idea that the light shielding member 25 intervenes can beapplied to the case where the wavelengths from the light sources 3 (Y,M, C and B) are different from one another and thus an optical pathcorrecting element 27 should be provided (see a third embodiment,mentioned later).

According to the optical multi-beam scanning device and the imageforming apparatus of the second embodiment, the light shielding memberas the edge portion is provided before the horizontal synchronizationsensor to which the light beams similar to those to be given to thesurface to be scanned are given. For this reason, even when latentimages are written on the surface to be scanned by the light beams whichtilt with respect to the surface to be scanned, the displacement in thehorizontal scanning direction can be suppressed. As a result, thequality of a formed image can be improved.

(C) Third Embodiment

FIG. 13 is a diagram illustrating only an optical path between thefolding mirror 29 and the horizontal synchronization sensor 23 in theoptical multi-beam scanning device according to a third embodiment whichis different form that in the first and the second embodiments. In thethird embodiment, the optical multi-beam scanning device has the opticalpath correcting element 27 shown in FIG. 16 between the folding mirror29 for guiding light to the horizontal synchronization sensor 23 and thehorizontal synchronization sensor 23. In the first and secondembodiments, the method of making the wavelengths from the light sourcesdifferent from one another can be combined with, for example, methods ofvarying an incident angle to the post-deflection optical element otherthan the method of making the scanning widths among the beams withdifferent optical paths uniform at the same deflecting angle. In thethird embodiment, however, when the wavelengths of the light sources 3(3Ya, 3Yb, 3Ma, 3Mb, 3Ca, 3Cb, 3Ba and 3Bb) for emitting beams forcreating the same latent images are made to be different, the scanningwidths among the beams with different optical paths are aligned with oneanother.

The function of the optical path correcting element 27 is explained. Theoptical path correcting element 27 is provided for the case adopting asystem such that the wavelengths from the light sources 3 (Y, M, C andB) are made to be different from one another and magnification chromaticaberration of the post-deflection optical system is canceled.

A case where when the incident angle α to the photosensitive drum 58 is15° and a scanning linear density in the vertical scanning direction is600 dpi, and two beams are scanned with a pitch of 42.33 μm isconsidered (see FIG. 14). A difference in the optical paths between thetwo beams is generally expressed by Δ=P×sin α on the optical axis (P isa distance between the light beams on the surface to be scanned, and αis the incident angle of the light beams to the surface to be scanned(photosensitive drum)). In this case, the difference between the opticalpaths Δ becomes 0.010956673 mm. When an angle (deflecting angle) at thescanning end is 30°, a shift of the beam position in the horizontalscanning direction due to the difference in the optical paths becomesΔ×tan (30 deg)=0.006325838.

FIG. 15 is a graph illustrating a change in the positions of the laserbeams passing through the paired imaging lenses 21 a and 21 b and areimaged on the imaging surface as a relative position in the horizontalscanning direction when the wavelengths of the laser beams emitted fromthe semiconductor laser elements (light sources) change. When the laserbeam with wavelength of 680 nm (curved line a) is used as a standard,positions of the laser beams with wavelengths of 665 nm (curved line b),670 nm (curved line c), 675 nm (curved line d), 685 nm (curved line e),690 nm (curved line f) and 695 nm (curved line g) in the horizontalscanning direction are shown.

According to the graph, when the light sources with a difference in thewavelength of 5 nm are used, the displacement of the beams due to thedifference in the optical paths and the displacement of the beams due tothe difference in the wavelengths can be canceled.

When such a difference in the wavelengths are set and the displacementsare canceled, the optical path correcting element 27 for making not thewavelengths but the beam positions on the horizontal synchronizationsensor 23 uniform is provided before the horizontal synchronizationsensor 23. As a result, even if the horizontal synchronization sensor 23is provided so as not to have a tilt in the horizontal scanningdirection and the vertical scanning direction similarly to theconventional manner, when the positions in the horizontal scanningdirection on the photosensor match with one another, the beams can cometo the detection stripe on the horizontal synchronization sensor 23.

The optical path correcting element 27 is provided on the optical pathbetween the imaging lenses 21 and the horizontal synchronization sensor23. It is composed of a prism or a diffraction grating which can changethe emitting angle according to a fluctuation in the wavelengths of thelaser beams form the light sources in the horizontal scanning direction,and can shift the beam positions with the same amount as and to anopposite direction to the displacement generated by the imaging lensesdue to the difference in the wavelengths. Even when the wavelengths ofthe laser beams fluctuate, the beams can be guided to the same positionon the detecting surface of the horizontal synchronization sensor 23.

As the optical path correcting element 27, a prism whose sectional shapeis an isosceles triangle shown in FIG. 16 can be used. As a method ofselecting various parameters in the case which adopts the prism shown inFIG. 16, a method disclosed in Japanese Patent Application Laid-Open No.11-194285 (1999) can be adopted.

The optical path correcting element 27 is provided so as to guide thebeams to the same position in the horizontal scanning direction on thedetecting surface of the horizontal synchronization sensor 23 regardlessof the fluctuation in the wavelengths. As a result, even when thedifference in wavelengths fluctuates due to mode hopping or the likecaused by temperature change while the lengths of the scanning lines arealigned, a shift of the relative printing position can be suppressed to½. As shown in FIG. 1, a plurality of beams are used in order to writefour latent images on the four photosensitive drums (or photosensitivebelts, four places on the photosensitive drum) 58 (Y, M, C and B). Evenwhen the angles in the vertical scanning direction at which the beamsenter the photosensitive drums are different from one another, theconstitution including the optical path correcting element 27 cansuppress the displacement of the beams in the horizontal scanningdirection even at the use of a common part or each set of parts.

FIG. 17 is a diagram illustrating a reflection type diffraction grating129 in which the function of the folding mirror 29 in FIG. 13 isintegral with the function of the optical path correcting element 27.The reflection type diffraction grating 129 leads the beams to thehorizontal synchronization sensor 23. Its function and effect are thesame as those when the optical path correcting element 27 is provided.

According to the optical multi-beam scanning device and the imageforming apparatus according to the third embodiment, the system ofmaking the wavelengths from the light sources different so as to cancelthe magnification chromatic aberration of the post-deflection opticalsystem is adopted. In this case, the optical path correcting element isprovided, so that the beams are emitted to the same position of thehorizontal synchronization sensor even when the wavelengths fluctuatedue to temperature. For this reason, the displacement in the horizontalscanning direction can be suppressed, and thus the quality of the formedimage can be improved.

(D) Another Embodiment

The technical idea that the optical path correcting element and thereflection type diffraction grating according to the third embodimentare provided can be combined with the technical ideas in the first andsecond embodiments.

The present invention can be widely applied to the optical multi-beamscanning device including the component for tilting the beams to thevertical scanning direction with respect to the normal direction of thesurface to be scanned and emitting the beams to the surface to bescanned, and the image forming apparatus including such an opticalmulti-beam scanning device. The present invention can be, therefore,applied to various apparatuses for color and monochrome modes and to anapparatus for the color mode adopting a constitution for scanning onlyblack with a plurality of beams.

1. An optical multi-beam scanning device, comprising: a plurality oflight sources; deflecting means for deflecting light beams from thelight sources; post-deflection optical means for making the light beamsdeflected by the deflecting means enter a surface to be scanned in avertical scanning direction with respect to a normal direction of thesurface to be scanned at a predetermined angle; horizontalsynchronization detecting means for synchronizing the light beams in ahorizontal scanning direction; and optical path folding means forfolding the light beams, directing towards the surface to be scanned, tothe horizontal synchronization detecting means; wherein an opticalelement for changing an emitting angle according to a fluctuation in thewavelengths of the light beams emitted from the light sources isarranged on an optical path between the deflecting means and thehorizontal synchronization detecting means.
 2. The optical multi-beamscanning device according to claim 1, wherein the optical element forchanging the emitting angle according to the fluctuation in thewavelengths of the light beams has a wavelength characteristic such thatpositions of the light beams on the horizontal synchronization detectingmeans do not change even when the wavelengths change.
 3. An imageforming apparatus, comprising: the optical multi-beam scanning deviceaccording to claim 1; and a photoreceptor having a surface to be scannedon which latent images are formed based on light beams from the opticalmulti-beam scanning device.